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Pneumatic Conveying of Solids

Pneumatic Conveying of Solids - Springer978-94-009-0405-7/1.pdf · 3.4 Equations for calculation of relevant properties 67 3.5 Fluidization characteristics of powders 78 ... experiences

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Pneumatic Conveying of Solids

Powder Technology Series

EDITED BY B. SCARLETT

Department of Chemical Engineering University of Technology Loughborough

Many materials exist in the form of a disperse system, for example powders, pastes, slurries, emulsions and aerosols. The study of such systems necessarily arises in many technologies but may alternatively be regarded as a separate subject which is concerned with the manufacture, characterization and manipul­ation of such systems. Chapman and Hall were one of the first publishers to recognize the basic importance of the subject, going on to instigate this series of books. The series does not aspire to define and confine the subject without duplication, but rather to provide a good home for any book which has a contribution to make to the record of both the theory and the application of the subject. We hope that all engineers and scientists who concern themselves with disperse systems will use these books and that those who become expert will contribute further to the series.

Particle Size Measurement T. Allen 4th edn, hardback (0 412 35070 X), about 736 pages

Powder Porosity and Surface Area S. Lowell and Joan E. Shields 2nd edn, hardback (0 412 25240 6), 248 pages

Pneumatic Conveying of Solids R.D. Marcus, L.S. Leung, G.E. Klinzing and F. Rizk Hardback (0 412 21490 3), 596 pages

Particle Technology Hans Rumpf Translated by F.A. Bull Hardback (0 412 35230 3), about 160 pages

Pneumatic Conveying of Solids

R.D. MARCUS University of Pretoria,

South Africa

L.S. LEUNG Commonwealth Scientific and Industrial Research Organization,

Australia

London

G.E. KLINZING University of Pittsburgh, United States of America

F. RIZK BASF AG, Ludwigshafen,

Federal Republic of Germany

CHAPMAN AND HALL

New York Tokyo Melbourne Madras

UK Chapman and Hall, 11 New Fetter Lane, London EC4P 4EE

USA Chapman and Hall, 29 West 35th Street, New York NY10001

Japan Chapman and Hall Japan, Thomson Publishing Japan, Hirakawacho Nemoto Building, 7F, 1-7-11 Hirakawa-cho, Chiyoda-ku, Tokyo 102

Australia Chapman and Hall Australia, Thomas Nelson Australia, 480 La Trobe Street, PO Box 4725, Melbourne 3000

India Chapman and Hall India, R. Sheshadri, 32 Second Main Road, CIT East, Madras 600035

© 1990 R.D. Marcus, L.S. Leung, G.E. Klinzing and F. Rizk Softcover reprint of the hardcover 1st edition 1990

Typeset in 10/12 Times by Thomson Press (India) Ltd, New Delhi ISBN -13: 978 -94-010-6669-3 e-ISBN -13 :978-94-009-0405-7 DOl: 10.1007/978-94-009-0405-7

All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, or stored in any retrieval system of any nature, without the written permission of the copyright holder and the publisher, application for which shall be made to the publisher.

British Library Cataloguing in Publication Data

Pneumatic conveying of solids 1. Solids. Pneumatic transport in pipes I. Marcus, R. D. II. Series 621.8'672

ISBN -13 :978-94-010-6669-3

Library of Congress Cataloging in Publication Data is available

Contents

Preface

Foreword

Nomenclature

x

xi

xiii

1 An overview of pneumatic conveying systems and performance 1

1.1 Introduction 1 1.2 Why pneumatic conveying? 1 1.3 What can be conveyed? 2 1.4 What constitutes a pneumatic conveying system? 6 1.5 Modes of pneumatic conveying 8 1.6 Basic pneumatic conveying systems 10 1.7 Further classification techniques 15 1.8 Description and operation of a pneumatic conveying system 16 1.9 Putting it all together 20 1.10 An overview 26 1.11 Some useful conversion factors and tables 26

References 33

2 Single phase flow in pneumatic conveying systems 35 2.1 Introduction 35 2.2 Definitions 35 2.3 Perfect gas laws 37 2.4 Drying of compressed air 37 2.5 The compression process 39 2.6 Gas flow through pipes 44 2.7 Illustrative examples 51

References 56

3 Fluid and particle dynamics 57 3.1 Introduction 57 3.2 Law of continuity 57 3.3 Drag on a particle 58

vi Contents

3.4 Equations for calculation of relevant properties 67 3.5 Fluidization characteristics of powders 78

References 80

4 Fundamentals 83 4.1 Introduction 83 4.2 Forces acting on a single particle in an air stream 83 4.3 Particle size 85 4.4 Shape 88 4.5 Dynamic equations 92 4.6 Terminal velocity 93 4.7 Single particle acceleration 93 4.8 Centrifugal flow 94 4.9 Slip velocity in a gravitational field 95 4.10 Multiple particle systems 96 4.11 Voidage and slip velocity 98 4.12 Frictional representations 103 4.13 Acceleration and development regions 106 4.14 Particle distribution in pneumatic conveying 107 4.15 Compressibility effect not negligible 108 4.16 Speed of sound in gas-solid transport 111 4.17 Gas-solid flow with varying cross-sectional area 113 4.18 Branching arrangements 115 4.19 Bend analysis 116 4.20 Downward sloping particle flow 120 4.21 Dense phase transport 122 4.22 Estimation of pressure drop in slugging dense phase

conveying 126 4.23 Estimation of pressure drop in non-slugging dense phase

conveying 129 4.24 Plug flows 134 4.25 Worked examples 143

References 156

5 Flow regimes in vertical and horizontal conveying 159 5.1 Introduction 159 5.2 Choking versus non-choking system in vertical flow 163 5.3 Choking system in vertical flow 172 5.4 Non-choking system in vertical flow 182 5.5 Particle segregation in vertical pneumatic transport 183 5.6 Saltation in horizontal conveying 185

References 189

Contents vii

6 Principles of pneumatic conveying 192 6.1 Introduction-putting it all together 192 6.2 The state diagram revisited 192 6.3 Methods for scaling-up 207 6.4 Use of theoretical models and definitions 209 6.5 Additional pressure drop factoz (A,z) 214 6.6 Pressure drop 216 6.7 Some important functional relationships 222 6.8 Sequence to be followed to obtain the system pressure loss

(Ap) 230 References 237

7 Feeding of pneumatic conveying systems 238 7.1 Introduction and overall design philosophy 238 7.2 Classification of feeding systems 239 7.3 Feeder selection criteria 239 7.4 Low pressure feeding devices 239 7.5 Medium pressure feeding systems 264 7.6 High pressure feeding devices 273 7.7 Conclusions 306

References 307

8 Flow in standpipes and gravity conveyors 309 8.1 Introduction-standpipes and gravity conveyors 309 8.2 Classification of standpipe systems 309 8.3 Classification of flow modes in a standpipe 314 8.4 Equations pertaining to each flow mode 318 8.5 Flow through a valve 320 8.6 Stability of standpipe flow 323 8.7 Analysis of industrial standpipes-case studies 327 8.8 Gravity conveyors 331

References 337

9 An overview of high pressure systems including long distance and dense phase pneumatic conveying systems 339

9.1 Introduction 339 9.2 High pressure systems 340 9.3 Dense phase flow classification 340 9.4 A description of plug flow and the relationships between

plug flow and material characteristics 341 9.5 System selection and product characteristics 345

viii Contents

9.6 Dense phase system design 347 9.7 Long distance pneumatic conveying and pressure loss

minimization 350 9.8 Conclusions 359

References 359

10 Gas-solids separation 361 10.1 Introduction 361 10.2 Selection criteria 361 10.3 Cyclone separators-theory of the separation of particles in

the centrifugal field 364 10.4 Fabric filters 391 10.5 Cleaning by sound 407 10.6 Conclusions 408

References 408

11 Some comments on: the flow behaviour of solids from silos; wear in pneumatic conveying systems; ancillary equipment 409

11.1 Introduction 409 11.2 The flow of solids from bins 409 11.3 Flow aid devices for silos and hoppers 420 11.4 Wear in pneumatic conveying systems 428 11.5 Ancillary equipment 440 11.6 Conclusions 453

References 453

12 Control of pneumatic transport 456 12.1 Basic material flow and control theory 456 12.2 Transport lags 459 12.3 Analysis of gas-solid flow by transfer functions 459 12.4 Stability of pneumatic transfer systems 462 12.5 Stability analysis with Taylor series linearization 463 12.6 Linear stability analysis-Jackson approach 465 12.7 Stability via the Liapunov analysis 467

References 469

13 Instrumentation 471 13.1 Standard instrumentation 471 13.2 Transducers 471 13.3 Cross-correlation procedures 473 13.4 A Coriolis force meter 475 13.5 Dielectric meter 476

Contents ix

13.6 Load cells 480 13. 7 Particle tagging 481 13.8 Electrostatic based meters 482 13.9 Acoustic measurements 485 13.10 Screw conveyors 487 13.11 Light measuring devices 488 13.12 Other techniques for particle velocities 491 13.13 Instrumentation for industrial applications 491

References 505

14 System design and worked examples 507 14.1 Introduction 507 14.2 Moisture content in air 507 14.3 The design of industrial vacuum systems 510 14.4 Dilute phase pneumatic conveying system design (method 1) 520 14.5 Dilute phase pneumatic conveying system design (method 2) 528 14.6 Dilute phase pneumatic conveying system design (method 3) 535 14.7 Dense phase pneumatic conveying system design 542 14.8 Test yourself-dilute phase calculations 546 14.9 Gas-solid flow examples 548 14.10 Conclusions 569

References 569

Index 570

Preface

When the four of us decided to collaborate to write this book on pneumatic conveying, there were two aspects which were of some concern. Firstly, how could four people, who live on four different continents, write a book on a fairly complex subject with such wide lines of communications? Secondly, there was the problem that two of the authors are chemical engineers. It has been noted that the majority of chemical engineers who work in the field of pneumatic conveying research have spent most of their time considering flow in vertical pipes. As such, there was some concern that the book might be biased towards vertical pneumatic conveying and that the horizontal aspects (which are clearly the most difficult!) would be somewhat neglected.

We hope that you, as the reader, are going to be satisfied with the fact that you have a truly international dissertation on pneumatic conveying and, also, that there is an even spread between the theoretical and practical aspects of pneumatic conveying technology.

We have attempted to produce a book for which we perceived a need in the market place. The book has been written taking into consideration our experiences in the pneumatic conveying industries, and also taking cognizance of the days when we started off as junior research workers in this field. In those early days, it was clear that a large amount of information pertaining to pneumatic conveying system design was not documented in the literature. Also, there was a certain amount of scepticism amongst the industry as to the effectiveness of research work carried out at universities. The fact that academics working in the field have been branded as 'one-inch' pipe technologists is indicative of the lack of confidence shown by industrialists.

As such, we have attempted to address both problems and to cater for a wide cross section of readers, including practising engineers, researchers, graduate students, plant operating personnel and the like. The text has been so arranged to accommodate those researchers who wish to gain more insight into the fundamentals, or to provide a short circuit for those readers who wish to address only the design issues relating to pneumatic conveying.

For the systems designer, it is recommended that Chapter 1 be consulted and, thereafter, the reader should study Chapters 6,7,9, 10 and 11. In Chapter 8 issues pertaining to the design of air-activated gravity conveyors are discussed, whilst a number of practical system design problems are solved in Chapter 14.

The researcher wishing to gain more insight into the technology is advised to

Preface xi

consult Chapters 3,4,5, 12 and 13. In these chapters an attempt has been made to review the relevant literature.

Those issues which are deemed important, but peripheral to this text, are discussed in Chapter 11, where an attempt has been made to alert the reader on such issues as silo and hopper design, wear and attrition, and ancillary equipment. All these topics are subjects in their own right, and the reader wishing to gain more insight into these aspects is advised to consult more definitive texts.

We hope you find this text both useful and stimulating, and that you will reap the rewards of entering into an exciting yet complex field of fluid mechanics.

Foreword

R.D. Marcus L.S. Leung

G.E. Klinzing F. Rizk

Pneumatic conveying is a most important practical operation whose application is a vital and integral part in the good design of many processes. Such a book can only be written from the basis of wide experience, in this case particularly achieved by the contribution and cooperation of four authors. The book combines a complete description of all the aspects of pneumatic conveying systems as well as the technology of feeding the systems and of separating the transported particles and the gas. A basically technological subject such as this can only be systematically explained from a basis of a good understanding of fluid mechanics and fluid particle interactions. This book also includes a description of all the necessary basic laws. It is a welcome addition to the Chapman Hall Powder Technology Series.

B. SCARLETT

Nomenclature

Symbol Description Unit

Ao Cross-sectional area of opening m2

A=nD2/4 Cross-sectional area of pipe m2

Constant or index for acceleration A* Cross-sectional area of a particle m2

normal to flow a Speed of sound in a pure gas mls as Speed of sound in a gas-solid mixture mls C1,2 Constant with index 1,2 c Average particle velocity, bubble mls

velocity, slug velocity Cc Particle velocity at choking mls

conditions c' Superficial solid velocity [ = c(1 - e)] mls

2F Co = pw2A* Drag coefficient

CD 00 Drag coefficient for undisturbed and unbounded fluid

CN Discharge coefficient for a nozzle Ct Volumetric concentration Cv Valve coefficient D Pipe inner diameter mm Do Diameter of orifice or valve opening m DB Bend curvature diameter mm d Spherical particle diameter mm dv Volume equivalent particle diameter mm {( Volume surface mean diameter mm dvs Specific surface area diameter = 4 d mm Ex Electric field N/C F Force N Fr = v/(Dg)1/2 Froude number related to air

velocity Fr* = c/(Dg)1/2 Froude number related to particle

velocity

Nomenclature xiii

Frf = wd(Dg)1/2 Froude number related to particle fall velocity

Fr: = C/(dg)1/2 Froude number related to particle velocity and particle size

fL Air friction factor G,aG Bulk solid mass, element of bulk kg

solid mass G Solid mass flow rate kg/s

kg/h t/h

Ga = papg(4dY/1'f2 Galileo number Ga' = papgd~/1'f2 Galileo number Gf=pv Gas flux kg/m2s Gs = G/A Solids flux kg/m2s g = 9.81 Acceleration due to gravity m/s2

(gravitational constant) g Index for gravitational H Enthalpy kcal/kg IR Relative turbulence intensity K,K1,K2 Constants k Effective pipe wall roughness mm L,aL Length, length of element m M Mass of gas kg mp Particle mass kg n Number of particles in an element,

Number of events or as exponent P Power of blower W,kW P Absolute pressure Pa,kPa Pdyn = pv2/2 Dynamic pressure Pa,kPa Po Atmospheric pressure Pa,kPa Pstat Static pressure Pa,kPa apg Gauge pressure or static pressure Pa,kPa ap Pressure difference Pa,kPa

Pa,kPa apz = ap! + apG Additional pressure drop due to Pa,kPa

presence of solids

a * - A*P* 2 aL Pressure drop due to impact and Pa,kPa pz- zTc D friction of solids

apG = p*gaZ Pressure drop due to gravity Pa,kPa Q Gas mass flow rate kg/s

kg/h ro Radius of pipe m R Gas constant Nm/kgk

xiv Nomenclature

RB Radius of curvature of bend m Re = vDlv Reynolds number related to pipe

diameter Rep = vdlv Reynolds number related to particle

diameter Repf = wrodlv Reynolds number related to terminal

velocity of particle Remf = vmindv/v Reynolds number related to minimum

fluidization velocity S Entropy kcal/K t Ambient temperature °C V=A,1L Volume of pipe element m3

T Absolute temperature K V = nd3/6 • p • Volume of particle m3

V=Qlp Gas volume flow rate m3/s

~ = Glp* Solids volume flow rate m 3/h m 3/s

V= VIA Average air velocity (superficial m 3/h mls

velocity) Vc Air velocity at choking conditions mls Vo Fluidization velocity mls ve = vic; Actual gas velocity in the voids mls

or velocity of porosity wave v* m Friction velocity of mixture mls Vmb Minimum fluidization velocity at mls

which bubbling occurs vmin Minimum fluidization velocity mls s Surface area of a particle m2

Vy Fluctuating fluid velocity in the mls direction of mean motion

v* s Friction velocity at saltation mls point

Vso Friction velocity at saltation point mls of a single particle

Vs Spin velocity mls Vth Gas velocity in the ventury throat mls W Work J w=v-c Relative velocity, slip velocity mls Wb Rate of rise of a single bubble or plug in mls

a fluidized bed Ws = Ve - C Slip velocity in a plug mls Wro Single particle settling (or terminal) mls

velocity in an undisturbed fluid

Nomenclature xv

Wf Particle settling velocity in a m/s cloud

W* Separation velocity m/s f

Slip velocity of a slug m/s Wsl X if Volume fraction of solid in feed

with terminal velocity WfOi

XiI Volume fraction of solid in tube with terminal velocity WfOi

Z,AZ Height, height difference m

Greek (alphabet) symbols

oc and OJ Indexes standing for initial and terminal states respectively

130 = wfO/v Velocity ratio related to single particle fall velocity

13 = welv Velocity ratio related to particle fall velocity in a cloud

f3w =tancpw Particle/wall friction factor y Gas/cloth ratio m/min ~ Exponent, internal angle of friction

p* e=1-- Voidage

pp

emf Voidage at minimum fluidization es Voidage at saltation velocity eo V oidage of fluidized bed ep V oidage of compact bed e V oidage of gas outside clusters 1'/ Dynamic viscosity Ns/m2 I'/B Blower, compressor efficiency I'/c Collector efficiency I'/F Fractional efficiency I'/s Separator efficiency () Angle degree A, = A,L + A,zJ.l Resistance factor of air and

solids in a mixture stream A,L Resistance factor for air alone

in pipe A,z Additional pressure drop factor in

a pipe due to solids in a flowing stream

A,zp Additional pressure drop factor due to air flowing through a plug in a pipe

xvi Nomenclature

2* z

Ji= G/Q

Ji* = IlG/IlQ Jiw v p P* = pp(l- e) IIp = Pp - P

atm B c C D dyn G H IF L mf o

stat V e

Pressure drop due to impact and fraction Pressure drop factor for slug flow Spatial macro scale between particles

{Mass flow ratio Mass load ratio

Mass concentration in pipe element Coefficient for sliding friction Kinematic viscosity Air (gas) density Apparent bulk density Density difference between particle and fluid Dust concentration Bulk density Mean density of mixture Particle density Relaxation time Shear stress Angle of wall friction Angle of internal friction Exponent Sphericity

Subscripts

Atmospheric conditions Bubble Particle Cluster Drag Dynamic Gravity Horizontal Impact and friction Gas Choking Conditions at atmospheric pressure Static Vertical Voidage

mm

m2/s kg/m3

kg/m3

kg/m3

mg/m3

kg/m3

kg/m3

kg/m3

s N/m2 degree degree